A hydraulic unit for a motor vehicle brake system of the type indicated is known from EP 0 950 004 B1. The hydraulic unit has, downstream of the outlet valves, a pump suction path to which a low-pressure accumulator is connected; a spring-loaded non-return valve via which the pump suction path is connected to a pump is arranged downstream of the low-pressure accumulator. Furthermore, the pump suction path is normally isolated, by means of a reversing valve which is closed in the rest position, from a pressure medium connection leading to a brake pressure sensor, it being necessary for the spring closing force on the non-return valve to be designed sufficiently large to reliably prevent pressure medium of the brake pressure sensor from flowing into the low-pressure accumulator as soon as the reversing valve is opened.
A translationally moved piston is received in a fluid-tight manner in a storage chamber provided in the low-pressure accumulator for storing the pressure medium, it being necessary, in order to ensure self-emptying of the low-pressure accumulator, for the actuating force of a piston spring to be designed large enough to be able to overcome, in addition to the piston friction, the relatively large closing force of the non-return valve spring arranged downstream of the low-pressure accumulator. During anti-lock braking control, the correspondingly strong spring-loading of the non-return valve prevents the formation of underpressure in the wheel brakes, with only low required brake pressures, while the pump is running.
The relatively large preloading force of the stiff piston spring has the disadvantage that, during anti-lock braking control, a high hydraulic actuating force is necessarily required in order that volume can be taken up by the low-pressure accumulator, which high hydraulic actuating force in turn impairs rapid pressure reduction in the wheel brakes. As already mentioned, the correspondingly strong spring-loading of the non-return valve represents a further obstacle.